Abstract: As a heat-resistant wave-absorbing material, silicon carbide (SiC) aerogel has become a research hotspot at present. However, the most common silicon sources are organosilanes, which are costly and toxic. In this work, SiC aerogels were successfully prepared by using water glass as the silicon source. Specifically, the microstructure and chemical composition of SiC aerogels were controlled by adjusting the Si to C molar ratio during the sol–gel process, and the effect on SiC aerogel microwave absorption properties was investigated. The SiC aerogels prepared with Si : C molar ratio of 1:1 have an effective electromagnetic wave absorption capacity, with a minimum reflection loss value of −46.30 dB at 12.88 GHz and an effective frequency bandwidth of 4.02 GHz. They also have good physical properties, such as the density of 0.0444 g/cm³, the thermal conductivity of 0.0621 W/(m·K), and the specific surface area of 1099 m²/g. These lightweight composites with microwave-absorbing properties and low thermal conductivity can be used as thermal protection materials for space shuttles and reusable carriers.